Biology Reference
In-Depth Information
Flexibility tends to be in short supply in purely physical morphogenesis. The simplest way
in which physical structures are built is self-assembly. Self-assembly is discussed in detail in
Chapter 3, because it is the essential foundation of biological morphogenesis too: all that
needs to be said here is that self-assembly is the coming together of subunits to make a struc-
ture because their association is energetically favourable and their association reasonably
probable (these conditions are explained further in Chapter 3). Pure self-assembly tends to
be relatively inflexible in terms of the range of structures that can be produced. Most
elements and compounds that can form crystals, for example, can form only one possible
structure. There is therefore no possibility for their modification by selection even though
crystals can 'reproduce' in a limited way (by being broken off and seeding further crystal
growth). Even where the same atoms or molecules can form more than one type of
crystal d for example, the way that carbon can form graphite or diamond and DNA can
form A-, B- and Z-type crystals d the form taken depends purely on the physical conditions
(such as pressure) during which crystallization takes place and has nothing to do with fitness
for any purpose.
Some physical processes, particularly ones that involve flows of energy, can produce
a variety of structures and do, therefore, have some degree of flexibility. An example of
such a physical process is provided by the formation of sand dunes where wind sweeps
across a desert. Any slight hillock in the sand, caused by random chance if nothing else,
creates a wind shadow to its leeward side. As air movement slows in this wind shadow,
blown sand grains carried in the air fall and the hillock grows, its peak moving a little to
leeward as it grows in height as the process repeats. The air in the lee of such a hillock of
sand carries few sand grains, not enough to amplify a hillock that is very close by, so only
at some distance from a growing dune may another dune begin to grow and thrive. The result
of these constraints is that windswept sand is sculpted into a series of spaced-out dunes, all
moving slowly to leeward across the desert. The height and spacing of the dunes depends on
the sizes of the grains, the speed of the wind and also the history of the system. As long as
wind energy continues to be available, morphogenesis continues. The system even involves
some degree of internal feedback, as the presence of an existing dune alters the probability of
sand deposition. Sand dune 'morphogenesis' is still intuitively different from biological
morphogenesis, though, because the form of the sand is dictated simply by the physical attri-
butes of the components involved and not by any feedback from how well the form is adap-
ted to function, because there is no function. )
Morphogenetic mechanisms of biology generally have another 'layer' to them that
provides negative feedback and adjusts morphogenetic processes to optimize them for
a specific function. For example, the morphogenesis of the cytoskeleton of an animal cell
(Chapters 4, 5 and 8) depends on the association of subunits by simple self-assembly, but
the probabilities of self-assembly and of disassembly at any place in the cell are modulated
by whether the structure at that place is fulfilling a useful structural role (for example, on
whether it is carrying mechanical tension or is flapping about doing nothing). Feedback of
this type arises when the output of a process (for example, the shape and location of
) This discussion assumes a desert devoid of life; for example, in the Martian desert. By stabilizing surface
sand, plants can modify sand dune morphogenesis to promote the development of dune systems optimized
for their needs.
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